1. Field of the Invention
This invention relates to a method and apparatus for determining characteristics, such as the actual or effective width of a conductive line or the precision of alignment of one line to another, particularly in regard to lines made by integrated circuit technology.
In the fabrication of integrated circuits in semiconductor wafers, conductive lines are formed within or over a semiconductor body, e.g., made of silicon. Conductive lines made within a semiconductor body are generally made by diffusing a dopant, such as boron or arsenic, in heavy concentration through a diffusion mask containing a given pattern, while conductive lines formed over a semiconductor body are made from a conductive layer of, e.g., aluminum or heavily doped polysilicon, disposed on an insulating layer interposed between the semiconductor body and the conductive layer. The width and length of the conductive lines formed on the insulating layer are determined, as is well known, by the pattern in a mask and photoresist and etch conditions employed.
When layouts are designed for semiconductor integrated circuits, an important consideration for successful operation of a given circuit is the actual or effective width of the conductive lines formed on or in the semiconductor body. As each layout is prepared, an optimum or design width is determined for each conductive line. This nominal width of the line can be attained only if perfect processing is employed throughout all manufacturing steps of the line. However, it is well known that masks produced for making integrated circuit lines often deviate from layout design specification due, e.g., to over or under exposure of photoresist during the mask making process. It is also known that even if the mask contains the nominal width for a desired line, over or under etching of the conductive layer produces lines that are either too thin or too wide compared with the nominal or design width. Lines produced with significant variations of width from the nominal width can cause reliability problems due to undesirable short or open circuits or resistances and, therefore, should be detected as soon as possible during the manufacturing process of an integrated circuit.
Another important characteristic of conductive lines used in semiconductor integrated circuits is the precision of their alignment with another element located on another layer or level. Lines having improper alignments can produce operational and reliability problems.
The determination of the amount of deviation of the actual line width from the nominal or design line width and the amount of misalignment can serve as a prediction at an early stage in the process of making integrated circuits of the success of the operability of the circuits.
2. Description of the Prior Art
Various systems have been suggested for determining the characteristics of lines, layers or bodies, particularly in semiconductor technology. In U.S. Pat. No. 3,650,020 a method employing a V-shaped mask pattern is described for monitoring the extent of lateral and vertical diffusion of regions of transistor elements during the production of integrated circuits, with oxide etching and photographic mask definition also being monitored during fabrication. The base width of a transistor is determined in U.S. Pat. No. 3,465,427, commonly assigned, by determining the sheet resistance of the base material, and in U.S. Pat. No. 3,440,715 by determining the current gain characteristics of test transistors. In commonly assigned U.S. application having Ser. No. 538,288, filed on Jan. 2, 1975, by D. R. Thomas, and entitled, "Conductive Line Characteristics Measuring System," there is described a system for determining etch bias by measuring voltage drops between a known distance along two conductors of different width but carrying the same or equal currents. Many well known optical systems have been used to determine line widths of conductive lines. In commonly assigned U.S. Pat. No. 3,808,527, filed on June 28, 1973, by D. R. Thomas, any misalignment of masks during the production of integrated circuits is determined by measuring appropriate voltages in a test circuit along an elongated resistor from a given contact point.